1. Field of the Invention
This invention relates to a multi-wavelength optical batch amplification apparatus.
2. Description of the Related Art
In an optical communication field in recent years, a technique of multiplexing and transmitting a plurality of input optical signals of different wavelengths has been and is being investigated energetically, and also development of a multi-wavelength optical batch amplification apparatus which amplifies a plurality of input optical signals of different wavelengths in a batch has been and is being performed rapidly.
Actually, however, the gain to input optical signal wavelength characteristic of an optical amplifier is not flat, and when a plurality of input optical signals having different wavelengths are amplified in a batch, the gains of the wavelengths of the input optical signals rely upon the total number, the wavelengths, the powers and so forth of input optical signals. Consequently, the optical amplifier exhibits a very complicated behavior.
Therefore, a multi-wavelength optical batch amplification apparatus is demanded which can amplify input optical signals having different wavelengths from each other in a batch so that the powers of output optical signals may equally exhibit a desired output value without being influenced by the conditions of the input optical signals.
FIG. 33 shows in block diagram a construction of an ordinary multi-wavelength optical batch amplification apparatus. Referring to FIG. 33, the multi-wavelength optical batch amplification apparatus shown includes a plurality of optical signal sources (E/O) 100-1 to 100-N (N is a natural number), a wavelength multiplexing (WDM) wave combiner 101, two couplers (CPL) 102 and 107, two isolators (ISO) 103 and 106, an erbium doped fiber (EDF) optical amplifier 104, a wave combiner 105, two photodiodes (PD) 109 and 110, a laser diode (LD) 111, and an automatic level control (ALC) circuit 112.
The optical signal sources 100-1 to 100-N output optical signals having different wavelengths .lambda.1 to .lambda.n (n being a natural number) from each other. The WDM wave combiner 101 combines the optical signals of the different wavelengths from the optical signal sources 100-1 to 100-N to multiplex them in wavelength. Each of the couplers 102 and 107 branches an optical signal inputted thereto.
Each of the isolators 103 and 106 removes, from an optical signal from the WDM wave combiner 101 or the wave combiner 105, noise components caused by an insertion loss of the WDM wave combiner 101 or the wave combiner 105 or by reflected light during transmission of the optical signal. The EDF optical amplifier 104 amplifies component optical signals of a wavelength multiplexed signal from the WDM wave combiner 101 in a batch to a desired power. The wave combiner 105 combines the output of the EDF optical amplifier 104 and the output of the laser diode 111.
Further, each of the photodiodes 109 and 110 produces an electric signal corresponding to the power of an optical signal branched by the coupler 102 or 107. The laser diode 111 generates pumping light to be combined with the output of the EDF optical amplifier 104 by the wave combiner 105. The ALC circuit 112 performs feedback control of the EDF optical amplifier 104 based on the outputs of the photodiodes 109 and 110 so that the output of the EDF optical amplifier 104 may be fixed.
In the multi-wavelength optical batch amplification apparatus having the construction described above, a plurality of optical signals having different wavelengths (.lambda.1 to .lambda.n) from the optical signal sources 100-1 to 100-N are multiplexed in wavelength by the WDM wave combiner 101 and inputted by way of the coupler 102 and the isolator 103 to the EDF optical amplifier 104, by which the component optical signals of the wavelength multiplexed optical signal are amplified in a batch to a desired power.
The optical signal amplified by the EDF optical amplifier 104 in this manner is combined with pumping light from the laser diode 111 by the wave combiner 105 so that a wavelength multiplexed optical signal (.lambda.1+.lambda.2+ . . . +.lambda.n) wherein the optical signals are individually amplified to the desired power is obtained by way of the isolator 106 and the coupler 107.
By the way, the optical signal branched by the coupler 107 is converted into an electric signal corresponding to the total power of the component optical signals of the output optical signal then by the photodiode 110, and the electric signal is inputted to the ALC circuit 112. The ALC circuit 112 controls the output of the laser diode 111 based on the output of the photodiode 110, that is, based on the total power of the output optical signal, to perform feedback control so that the output of the EDF optical amplifier 104 may be fixed. It is to be noted that also the optical signal branched by the coupler 102 is converted into an electric signal in accordance with the power of the optical signal by the photodiode 109, and the electric signal is inputted to the ALC circuit 112. In this instance, the electric signal is supplied as input light interruption information of the output of zero when an input optical signal or signals are interrupted.
However, with the multi-wavelength optical batch amplifier described above, since the power control of an output optical signal is formed by way of ALC control of the total power of the component optical signals of the output optical signal having the wavelengths from .lambda.1 to .lambda.n, when some of the input optical signals enter into an interrupted condition, the output optical signal varies per one wave by EQU .DELTA.P=10.multidot.logK/(K-F)!(1)
where K is an initial number of signals, and F is a number of interrupted signals. In this instance, it is assumed that the EDF optical amplifier 104 has no wavelength characteristic.
In particular, for example, when three optical signals of wavelengths .lambda.1 to .lambda.3 are being outputted equally with a desired output power as seen in FIG. 34(a), if the input optical signal of the wavelength .lambda.3 enters into an interrupted condition, then the powers of the other output optical signals of the wavelengths .lambda.1 and .lambda.2 rise as seen in FIG. 34(b). Then, if also the input optical signal of the wavelength 12 enters into an interrupted condition, then the power of the output optical signal of the wavelength .lambda.1 further rises as seen in FIG. 34(c). Consequently, no output optical signal of the desired power can be obtained any more.
Also multi-wavelength optical batch amplification apparatus have been proposed which can suppress such variation of the output optical signal power per one wave as described above. An exemplary one of multi-wavelength optical batch amplification apparatus of the type just mentioned is shown in FIG. 35. Referring to FIG. 35, the multi-wavelength optical batch amplification apparatus shown includes, in addition to a plurality of optical signal sources 100-1 to 100-N, an optical signal source 100-M (M&gt;N) which outputs a control optical signal having a wavelength .lambda.m (m&gt;n). When one of the input optical signals enters into an interrupted condition, the variation of the output optical signal power per one wave is suppressed using the control optical signal from the optical signal source 100-M. It is to be noted that, in FIG. 35, those elements denoted by like reference characters to those of FIG. 33 are similar to those described hereinabove with reference to FIG. 33.
However, the multi-wavelength optical batch amplification apparatus described hereinabove with reference to FIG. 33 as well as the multi-wavelength optical batch amplification apparatus described hereinabove with reference to FIG. 35 are disadvantageous in several points. In particular, the multi-wavelength optical batch amplification apparatus shown in FIG. 33 is disadvantageous in that it is low in resisting property to a power variation (including an interrupted condition) of input optical signals. In particular, if all of the input optical signals of the wavelengths from .lambda.1 to .lambda.n exhibit variations by an equal level, then the output powers of the component optical signals of the output optical signals can be made equal to each other to some degree, but in any other case, it is almost impossible to make the output powers of the component optical signals of the output optical signal equal to each other.
Meanwhile, with the multi-wavelength optical batch amplification apparatus shown in FIG. 35, the output power variations of component optical signals of an output optical signal which occur when some of input optical signals enter into an interrupted condition can be suppressed to some degree using the control input optical signal (wavelength .lambda.m) from the optical signal source 100-M. However, if the control optical signal enters into an interrupted condition, then the power control of the output optical signal is still disabled.
It seems a possible idea to use the control optical signal of the wavelength .lambda.m as an optical signal for exclusive use for controlling the output optical signal power. However, from the object of wavelength multiplexing which is employed in order to assure an enlarged transmission capacity, it should be avoided to use one wavelength only for output control in this manner.
Further, the multi-wavelength optical batch amplification apparatus are effective only when it is assumed that, upon multi-wavelength batch amplification by the EDF optical amplifier 104, the gain tilt thereof is free from input optical signal wavelength dependency, input optical signal power dependency and pumping light power dependency. In any other case, however, it is very difficult to control the output optical signal powers of different wavelengths strictly equal to each other.